The present invention related to the video arts. It finds particular application in conjunction with adapting lower resolution interlaced television signals for display on higher resolution CRT displays of computer monitor and will be described with particular reference thereto. It is to be appreciated, however, that the invention will find other applications in the video signal processing field, such as adapting high resolution television monitors for use with low resolution broadcast signals, for zooming or magnifying video images, for converting video signals of one resolution to video signals of another resolution, and the like.
Television signals commonly comply with the National Television System Committe (NTSC) standards which call for 525 interlaced raster lines. The raster lines are scanned with a horizontal frequency of 15.75 kHz and a vertical frequency of 60 Hz. The odd raster lines, i.e. the 1st, 3rd, 5th, etc. alternate lines of the image are displayed alternately with the even raster lines, i.e. the 2nd, 4th, 6th, etc. alternately lines.
While this resolution has commonly been found acceptable for television images, higher resolution monitors are required to display computer data clearly and legibly. Although the resolution provided by the monitors varies, most computer monitors provide about 700 to 800 raster lines, a horizontal frequency nearly double the NTSC standard, and a vertical frequency of 60 Hz. That is, computer monitors commonly provide about 11/2 times as many dots or pixels per raster line and about 11/2 times as many raster lines per image. To fill the higher resolution computer monitor, the lower resolution video image must be expanded.
Heretofore, various interpolation schemes has been utilized for expanding the image. To expand the width of the image, about half of the dots were displayed more than once. For example, the even dots might be displayed once and each odd dot displayed twice. To increase the height of the image, alternate raster lines might be displayed twice. For example, in the odd field scan, the first line could be displayed, the third line could be displayed twice, the fifth line could be displayed, the seventh line could be displayed twice, and the like. Analogously, the even line field could be expanded to present: line 2, line 4, line 4, line 6, line 8, line 8, etc. One problem with this technique is that it weighted half of the lines more heavily than the other and caused a non-linear stretching of the image.
In another interpolation technique, the lines of the odd field were selectively averaged and the lines of the even field were selectively averaged. More specifically, after every two actual raster lines, a 3rd raster line was created by averaging the immediately preceding and the immediately following raster lines of the even or odd field. For example, the odd field might include line 1, line 3, the average of lines 3 and 5, line 5, line 7, the average of lines 7 and 9, etc. Analogously, the even field might include line 2, line 4, the average of lines 4 and 6, line 6, etc. One problem with this technique is that when the odd and even fields were interlaced, some of the data was displayed out of order. Specifically, in the resultant composite image or picture the line which was the average of lines 3 and 5 would be displayed below line 4. This placed some of the data from line 3 below line 4. Analogously, the next lowest line, the average of lines 4 and 6 placed data from line 6 above line 5.
Another interpolation scheme which kept the data in order required intermixing some data from the even and odd fields. More specifically, between pairs of lines of the odd field, the next line in sequence from the even field would be inserted. For example, the odd field might display lines 1, line 3, line 4, line 5, line 7, line 8, etc. Analogously, the even field might display line 2, line 4, line 5, line 6, line 8, line 9, etc. Although this technique kept the data in order, some of the lines were again displayed more often than others weighing the data from those lines more heavily.
The present invention provides a new and improved method and apparatus for converting video signals of a first resolution to video signals of another resolution.